THE GAME:
Nerds: Players use dice to roll character attributes, then take them on quests into often silly fantasy worlds for hours at a time. Scoring comes from a combination of luck and strategy. No actual fighting is done.
Jocks: Players draft characters over the web and relentlessly track the players on their own team. Scoring comes from a combination of luck and strategy. There is no actual playing of the sport.

THE TIME COMMITMENT:
Nerds: RPGs can be a total time-suck, often taking up entire evenings.
Jocks: Fantasy sports can be just as bad. Serious players spend days and sometimes weeks studying before the draft by following training camp, buying special magazines and reading dedicated websites. The draft generally takes a few hours on its own and the amount of time spent checking stats, watching games and bothering everyone in the league of the the messageboard is almost impossible to measure.

THE UNEARNED PRIDE:
Nerds: They can sit on their fat asses bragging about the conquests that happened in a land with a name I can't even pronounce.
Jocks: They sit on their slightly less-fat asses bragging about the on-field conquests that they played absolutely no part in.

IMPACT ON GIRLS
Nerds: Most girls are not impressed by guys who play RPGs. Bragging about how sweet your new two-handed sword +2 is will almost never help you get lucky. If it does, chances are it will be with a girl who calls herself Princess Moonbeam.
Jocks: Like other similar games, fantasy sports can be relationship poison. She is eventually going to get sick of hearing about how your pitcher gave up 3 in the bottom of the eighth and go fool around with your brother.

To sum it up, they're both fantasy games and if you take either one of them too seriously you will become insufferable to be around. So next time you stand around the water cooler arguing about which batter has a better slugging percentage with runners in scoring position, remember that to people who aren't in your league, you might as well be talking about how sweet it was when your level 8 warrior took down those orcs in the cave of neverending darkness.